It is well-known to tackle time-varying noise in multi-tone data transmission systems, i.e. transmission systems wherein data bits are modulated on plural tones or carriers to be conveyed between a transmitter and receiver, through various online reconfiguration mechanisms. These online reconfiguration mechanisms for instance include bitswap, Seamless Rate Adaptation (SRA) and Save Our Showtime (SOS) mechanisms.
A bitswap mechanism moves bits from tones with increasing noise to other tones without impact on the overall bitrate between transmitter and receiver.
Seamless Rate Adaptation adjusts the overall bit rate between transmitter and receiver downwards or upwards depending on the noise margin variation. In addition, the bitloading of the tones is adjusted in function of the changed overall bit rate.
Save Our Showtime is applied in case of a strong noise increase at a point in time where the noise margin of the multi-tone system is negative. Since it is impossible to accurately determine how many bits each carrier can convey, the bitloading is drastically decreased to avoid resynchronization or re-initialization of the multi-tone system. SOS is usually followed by SRA for upwards adaptation of the overall bit rate.
The above described online reconfiguration mechanisms are disadvantageous in various aspects. Online reconfiguration must first be negotiated between transmitter and receiver through a signalling protocol. Also the timing of online reconfiguration must be synchronised between transmitter and receiver. This may for instance be realised through a flag on the SYNC symbol on DMT based DSL systems. As a consequence, online reconfiguration is rather slow. In 4 kHz DMT based DSL systems, i.e. DSL systems that use a set of 4 kHz spaced apart carriers, the expected update time for online reconfiguration mechanisms is not below 128 DMT symbols or 32 milliseconds. As a result, transmission errors within the first 32 milliseconds after a noise transient must be corrected through mechanisms different from online reconfiguration. For SRA in VDSL, generation of the EOC or Embedded Operations Channel message, transmission of the EOC message, interpretation of the EOC message, and waiting for the SYNC symbol may involve a total reaction time of up to 400 milliseconds. When repeated partially for every group of 128 carriers saved through SOS, the online reconfiguration from transient to full-band execution may even occupy up to 1 second.
A fast way to correct transmission errors consists in retransmission of the erroneously received data transmission units. Thereto, the transmitter is equipped with a retransmission buffer that stores recently transmitted data transmission units. Upon receipt of an erroneous data transmission unit, the receiver requests retransmission through an Automatic Repeat request (ARQ). Upon receipt of the ARQ, the transmitter sends the stored copy of the data transmission unit to the receiver.
Although a retransmission scheme can be used to deal with noise transients in multi-tone data transmission systems up to the point in time where the bitloading has been adjusted through online reconfiguration mechanisms, retransmission is limited in capacity and introduces substantial overhead. Due to memory cost constraints and standardization requirements, the retransmission buffer typically stores about 10 milliseconds of transmitted data. In case of a strong noise transient, this may be insufficient to correct all transmission errors within the time interval required for online reconfiguration. As a result, traditional retransmission in combination with online reconfiguration may be unable to avoid line instabilities such as resynchronizations or re-initializations. Further, traditional retransmission of multi-tone symbols inherently impacts the instantaneous throughput of the line dramatically since the retransmitted data symbols occupy all or nearly all remaining bandwidth in case of a strong noise transient.
European Patent Application EP 1 011 245 entitled “Transmitter and Receiver and Data Transmission Method” describes hierarchical data modulation and data demodulation in single carrier communication systems (QPSK, BPSK, 16QAM based). In order to improve transmission efficiency, the receiver performs hierarchical demodulation, i.e. data transmission units named cells in EP 1 011 245 are demodulated from particular hierarchical noise levels. Thereafter, the receiver performs error detection and requests retransmission of an erroneous cell demodulated from a particular hierarchical level. The transmission efficiency is increased since the amount of data that must be retransmitted is reduced.
Although FIG. 10 of EP 1 011 245 and embodiment 4 described in paragraphs [0077]-[0081] of EP 1 011 245 disclose a multi-carrier system, each hierarchical modulation layer, e.g. hierarchy 1, hierarchy 2, or hierarchy 3, is tied to a distinct carrier, e.g. sub-carrier A, sub-carrier B or sub-carrier C. As a result, each cell or data transmission unit is still conveyed over a single, distinct carrier. A data cell in other words is assigned to a particular hierarchical level and as a result thereof also to a particular carrier. For this reason, EP 1 011 245 does not guarantee a stable line with optimized instantaneous throughput for multi-carrier systems in case of strong noise increases.
It is an objective of the present invention to disclose a system and method that improves the stability and instantaneous throughput of multi-tone data transmission under time varying noise. More particularly, it is an objective of the present invention to disclose an improved data retransmission request device, a data transmitter and a data retransmission method for use in multi-tone data transmission systems that resolve the above mentioned drawbacks of existing solutions.